A multi-level modulation technology is well known as a scheme for propagating a plurality of bits per symbol in view of increasing propagation amount per time in the radio communication system. In the multi-level modulation technology, as the more the number of bits per symbol increases, the more the maximum throughput under the good propagation channel quality. However, a problem arises that an error may be generated easily when propagation channel quality is lowered, and thereby the throughput is remarkably lowered. Therefore, in order to ensure a stable communication, an adaptive modulation technology has been proposed, in which a modulation scheme is switched in accordance with propagation channel quality, namely a modulation multi-level value is set to a higher value when the propagation channel quality is high and the modulation multi-level value is set to a lower value when the propagation channel quality is low. This adaptive modulation technology has been explained in the “Modulation Level Controlled Adaptive Modulation System with Base-Station-Based Transmission/Reception Diversity Scheme for Personal Communications” Shinya Otsuki, et al., IEEE Global Telecommunications Conference 1995 Proceedings Vol. 1537-41) or the like.
In addition to the modulation scheme in accordance with the propagation channel quality, a technology has also be proposed in which communication can be made with the throughput in accordance with the propagation channel quality by switching a coding rate of channel code. This technology has been introduced, for example, to a system of the standard specification, 3RD GENERATION PARTNERSHIP PROJECT 2, “cdma2000 High Rate Packet Data Air Interface specification”, 3GPP2 C. S0024-A Version 1.0, pp 14-21 to 14-23, 14-60 to 14-64, March 2000). Further, the system described in this non-patent document utilizes the technology called HARQ (Hybrid Automatic Repeat reQuest) in which a transmitter station previously encodes signals in lower encoding rate and transmits a part of the encoded signals, while a receiver station decodes the received signals, moreover, when the channel decoding is completed in successful, the transmitter station terminates transmission, if channel decoding fails here, the transmitter station transmits again another part of the encoded signals and the receiver station conducts channel decoding by combining this received signal with the signals received previously, whereby communication can be made in the throughput in accordance with the propagation channel by adjusting the encoding rate in accordance with the actual propagation channel quality.